Motorcycle Carburetor Theory 101

Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (above the piston on a four stroke). This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air.

Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.



Most motorcycle carburetor circuits are governed by throttle position and not by engine speed.There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:
* pilot circuit
* throttle valve
* needle jet and jet needle
* main jet
* choke circuit

The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.



The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).



The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.



The main jet controls fuel flow from 3/4 thru full throttle, fig 5. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the mainjet, the more fuel that can flow through it and the richer the mixture.



The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.

The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer. Figure 6 shows the actual air/fuel ratio for any given throttle opening.



Carburetor Jetting Troubleshooting

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.



While jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting.

Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this). After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color (for more info on reading spark plugs click here). If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture.

The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.


Altitude, Humidy, and Air Temperature

Once the jetting is set and the bike is running good, there are many factors that will change the performane of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temerature drops, the engine will run leaner and more fule will have to be added to compensate. When the air temerature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32š fahrenheight may run poorly when the temperature reaches 90š fahrenheight.

Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.

Humidy is how much moister is in the air. As humidy increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct coreection factor is found. Using fig 8 as an example, the air temperature is 95š fahrenheight and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.



Correction factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart from fig 9 and determin the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.

Needle Jet/Jet Needle/Air Screw Correction Chart


Correction factor

1.04 or above




0.92 or below

Needle jet

Two sizes larger

One size larger

Same size

One size smaller

Two sizes smaller

Jet needle setting

Lower clip position




Raise clip one position

Air screw opening

One turn in

1/2 turn in


1/2 turn out

One turn out





Date Last Updated 10-7-00

For private non profit use only. All rights reserved.

Please Contact me for use of this material. URL:



-         Pete Snidal, (C) 2000


Whether it's an Amal, - Standard, Monolbloc, Concentric, TT, or GP - or a Del'Orto, Mikuni, or Micarb, there's a sameness about slide-type carburetors as found on motorcycles built during the past 60 years and more. Air is admitted into the intake tract by means of raising a slide in the body of the carburetor, and consequently raising a fuel control needle attached to, and usually inside, the slide.

Fuel is maintained at a constant level in an adjacent float chamber, usually part of the carburetor body, by means of a float and valve arrangement, and is admitted into the airflow beneath the slide through a series of jets, the most important of these being the needle jet - the one the tapered needle slides in when the slide is moved. The size of this jet, and the taper and relative position of the needle are the determining factors for mixture (air/fuel ratio) in the range of 1/4 to 3/4 throttle, give or take. Once the throttle is above 3/4, the needle jet passes pretty well all the fuel that comes its way. It is regulated only by the size of the restriction below the needle jet, which is the main jet. Changing the size of this jet thus controls mixture at the "top end" of the throttle, and to an extent, the rpm range. (Since mixture is dependent to a degree on airflow, the upper end of the throttle range wants to be tuned at the upper end of the operating rpm range.

To set the jetting for either of these ranges, we do a "plug chop." The tip of the spark plug insulator changes colour with mixture - a white tip shows too lean a mixture, a black one, too rich. The ideal is just the sandy side of chocolate brown.

To do a plug reading, you want to run at 1/2 throttle for a minute or so, pulling upwind or up a hill, say, and then suddenly hit the neutral finder, if so equipped, or pull in the clutch, and turn of the key, coasting to a stop at the side of the road. Where you take the plug wrench out of your pocket, and pull out the plug and have a look. White plug, clip down, black plug, clip up.

For Main Jet tuning, repeat as above, but at full throttle. White plug, bigger jet, black plug, smaller jet. If it won't get up onto full throttle because of bucking and missing, you have to look at your pipe and if you see black smoke, you know it's 'way too big, so you'll have to try a smaller one. If it's too small, you'll get a white plug tip on full throttle, if not a lot of starving, bucking, missing, spitting back from the carb, (classic weak mixture) etc. Your main jet will have to be an awful lot too big before its size will affect the mid-range of the needle adjustment.

The needle position should already be set by this time. In extreme cases, you may not have been able to get a rich enough needle setting, because your mainjet may have been too small.

Variations in main jet size shouldn't affect needle setting, although if I've made big main jet changes, I always check the needle setting again.

The first 1/8 of throttle opening is affected by the pilot, or idle jet setting, and throttle cutaway affects the "transition" from idle circuit to needle. Adjust pilot jet at idle, alternating between the idle stop screw to get the lowest possible idle, and the pilot jet screw, to bring the idle up as much as you can. Throttle cutaway can only be changed by changing your slide, but fortunately, this is an adjustment that is seldom necessary. The indication that you need to will be hesitation (or worse) as you increase throttle from just above 1/8 to the needle range.

That's about all there is to getting your carb tuning "in the ballpark." Do remember that too lean a mixture will bring about overheating, often detonation and subsequent holing of the piston(s).


Follow steps in order....First, dial in:

CV Carb Tuning

Lower rpm engines,CV,lower_rpm_engines.html

§         Select Best Main Jet

o        To get the best, most even top end power (full throttle/after 5000 rpm), select the main jet that produces the highest top speed / pulls hardest at high rpm.

§         If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.

§         If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.

§         In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!

§         Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.

§         Select best needle clip position

o        To get the best power at full throttle / 2500-3500 rpm, adjust the needle height, after you have already selected the best main jet.

§         If the engine pulls better or is smoother at full throttle/2500-3500 rpm in a full throttle roll-on starting at <1500 rpm when cool but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered.

§         If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 2500-3500 rpm.

§         If the engine pulls equally well between 2500-3500 rpm when cooler as compared to fully warmed up, the needle height is probably properly set.

§         Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.

§         Float height (AKA fuel level & how to..)

o        To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling,  in 2nd gear from 1500 to 2000 rpm at minimum.

§         Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.

§         If the engine has a "wet" rhythmic, soggy area at full throttle / 1500 to 2000, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 1500 to 2000 leaner.

§         If the engine is "dry" and flat between 1500 to 2000 rpm, raise the fuel level.

§         Example: change float height from 15mm to 14mm to richen up that area.

§         REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.

§         Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.

§         Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.

§         Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!

§         If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs. Click here

§         Fuel Screw setting (AKA mixture screws)

§         There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil.  Cap removal details. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recal kit. We do have them available separately, too. 800 869-0497 to order -

§         Set for smoothest idle and 2nd gear, 2000 rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear, 2000 rpm steady state cruise , and 1/8 throttle high rpm operation.  (pj tuning information) 

§         Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!) AND pilot jet size are the primary sources of mixture delivery during 2000 rpm steady state cruise operation.

§         If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.

§         Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.

§         NOTE: A rich problem gets worse as the engine heats up.

§         If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.

§         NOTE: A lean problem gets better as the engine heats up.

§         If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!


Carburetor Jetting



Fundamentally, increasing fuel flow means installing larger jets in the carburetor. Sounds simple, doesn't it? Trust me, I've been doing it now for more than 30 years, and it really is that straightforward.


Picking the right jet, now that's the hard part. Fortunately that element of the job has been made much easier by the proliferation of jet kits now available. These kits contain all of the bits and pieces you'll need plus detailed instructions, and they are available to fit nearly every current and not-so-current motorcycle I can think of. But this would be a damn short article if we left it at that.


Rather than just show you how to install a jet kit, let me walk you through the basic steps used to jet a bike from scratch. Even though the kits greatly shortcut the jetting process, individual circumstances may dictate some fine-tuning from bike to bike. If you know how to evaluate jetting, you'll know what to look for and what changes need to be made. Since the most popular carburetor in use is the variable-venturi, slide-type carburetor, that's the model we'll use. For the sake of convention we'll assume you have either installed a jet kit and want to make sure the jetting is spot-on or you suspect some jetting changes need to be made.


Presumably you already have some fundamental understanding of a carburetor's internal workings. If you don't, I suggest you bone up on a little theory first. Mr. Bernoulli was a pretty bright guy, and his insights are well worth discovering.


It Just Ain't Running Right


Deciding where the problem lies is the first step. One of the more common mistakes the novice tuner makes is trying to jet based on engine rpm. Throttle position dictates which carburetor circuit is being used at the time, so jetting is always predicated by throttle position, never rpm.


Experienced tuners and riders are pretty good at determining which carburetor circuit is at fault. If you're new to this, a visual aid may prove helpful. A throttle index will show you exactly which circuit is in play at any given time. Using masking tape, place a datum point on the throttle housing. Next use another piece of tape to divide your throttle into quarters. As an alternative you can mark the quarters on the housing and use a colored pin stuck into the grip to indicate the throttle's position.


Your markings should indicate 1/4, 1/2, 3/4 and wide-open throttle, although the last is usually pretty obvious, as is the closed position.


Too often a carburetor jetting chart seems to indicate a very clear delineation among the effects of the various jets and circuits. There isn't; in fact, there is always some overlap, and bear in mind that carburetor fuel-delivery circuits are cumulative. In other words, even when we're running wide open on the main jet, the pilot jet is still supplying some fuel. For this reason we start by addressing the pilot jet first and the main jet last.


The Beginner's Guide to Jetting


Pilot circuit; 0 to 1/4 throttle


Start by bringing the engine up to operating temperature. Open the throttle slightly. Does the engine pick up speed smoothly? If it doesn't, the fuel-mixture screw may be out of adjustment or the pilot jet may be either too small or too large. To determine the pilot screw setting, adjust the idle-speed screw so that the engine is idling slightly fast, somewhere between 10 and 20 percent above the recommended idle speed. Using either the manufacturer's or the jet kit's recommended pilot screw setting as a starting point, turn the fuel screw left or right between 1/4 and 1/2 of a turn. Select the screw position where the engine speed increase is the greatest. Readjust the idle speed to the correct level.


Next, take the bike for a ride, preferably on a relatively quiet, straight section of road. Using your throttle index, run the bike at 1/4 throttle. If the pilot jetting is correct, the engine should run smoothly, and it should be easy to maintain a constant speed without varying the throttle position. If the bike surges or hunts, the pilot jet is probably too small (lean). Other indications that the pilot circuit is lean are popping or spitting through the carburetor when the throttle is opened and popping or backfiring through the exhaust when the throttle is shut. If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without "blipping" the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.


Low- to high-speed transition; 1/4 to 3/8 throttle


Although the throttle cutaway plays a significant role between 1/4 and 3/8 throttle, it is seldom changed when rejetting a streetbike, particularly a cruiser. For that reason I'll gloss over it. Basically the only time the cutaway needs changing is when the transition from the low-speed fuel circuit (pilot jet) to the high-speed circuit (needle, needle jet and main jet) gets seriously out of whack.


Medium throttle opening; 1/4 to 3/4 throttle


The fuel delivery at this stage is controlled by the jet needle and the needle jet. At one time all carburetors came with adjustable jet needles. Once the smog Nazis got involved, fixed-position needles became the standard. Jet kits will usually include new needles that can be raised (to richen mixture) or lowered (to make it leaner) to suit or, less often, shims to change the height of your stock needles.


To evaluate the needle/needle-jet performance, run the motorcycle in second or third gear. Roll the throttle on from 1/4 to 1/2 throttle only. The engine should accelerate cleanly without sputtering or bogging. If so, you're all done with the midrange; if it doesn't, you'll need to do some experimenting to determine if the needle position is too rich or too lean. Assuming you've installed adjustable needles from the kit, begin your experiments by trying some alternative needle-clip positions, perhaps a step richer to begin with. If that makes the situation worse, try a step leaner. If the jetting is stock and problems crop up, it's time to order the appropriate jet kit.


Top end; 3/4 to wide-open


At this stage of the game, the majority of the fuel flow is controlled by the main jet (remember some fuel is still flowing through the pilot jet). The time-honored method of determining main-jet size is via a plug chop. If you've never done one, a plug chop is pretty simple. Start with new spark plugs. Run the bike flat out (full throttle), in as high a gear as possible, for eight to 10 seconds (don't hold the bike at the rev limiter, as it'll give you a false reading). Without cutting the throttle or slowing down, simultaneously pull in the clutch and hit the kill switch. Coast the bike, with the clutch still in, onto the shoulder and remove the plug(s). The plug insulator should be tan to grayish-white. A white plug indicates a lean mixture; a dark gray or black one means it's rich. You then select the main-jet size accordingly.


Unfortunately, plug chops present a few problems. Some bikes require an inordinate amount of work to remove the plugs—at least more than you want to do on the side of the road with a smoking-hot bike. And in most parts of the country, the law takes a very dim view of banzai runs down the main drag followed by an impromptu tuning session on the side of the road.


The alternative, and the method I personally prefer, is to use a dyno. It's certainly a lot easier and arguably more accurate, but it may not be convenient.


Is It Lean or Rich?


One of the reasons I'm so stoked about jet kits is because they remove 95 percent of the guesswork. Be that as it may, even the best kits require some fine-tuning. Essentially a jetting problem can always be reduced to one of two choices: Is it lean or is it rich?


To determine if the bike is lean (to little fuel for the amount of air reaching the engine through the carburetor) at any given throttle opening, partially cover the air-filter intake with a piece of duct tape; if the carburetion improves, it's running lean. If you suspect the bike is running rich (too much fuel in the air-fuel mixture)), remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich.


If your bike is equipped with a manual fuel shut-off (or you're clever enough to disconnect and plug the fuel lines), take the bike for a ride with the fuel turned off. If it runs worse as the float bowl drains, it's lean. If it gets better, it's rich.




A lean condition is the end result of too little fuel and too much air. Slightly lean conditions create drivability problems. Worst-case scenario: Lean conditions can and do destroy engines. Holes in the pistons, burnt valves and trashed main bearings are the direct result of lean mixtures.


Rich mixtures are the end result of too much fuel and too little air. Rich mixtures waste fuel, contribute to carbon buildup and pollute the air.


Typical Lean Conditions:


- Poor acceleration; the engine feels flat.


- The engine won't respond when the throttle is snapped open, but it picks up speed as the throttle is closed. (A too-large main jet also mimics this symptom.)


- The engine runs hot, knocks, pings and overheats.


- The engine surges or hunts when cruising at part-throttle.


- Popping or spitting through the carb occurs when the throttle is opened. Or popping and spitting occurs through the pipe on deceleration with a closed throttle.


- The engine runs better in warm weather, worse in cool.


- Performance gets worse when the air filter is removed.


Typical Rich Conditions


- Engine acceleration is flat and uneven and loses that "crisp" feel.


- The engine "eight-strokes" as it loads up and skips combustion cycles.


- The engine's idle is rough or lumpy, and the engine won't return to idle without "blipping" the throttle.


- The throttle needs to be open continuously to maintain acceleration.


- Black, sooty plugs, a sooty exhaust pipe and black smoke from the tailpipe that stinks of unburned fuel.


- Poor fuel economy.


- The engine works better when cold. Performance falls off as it warms


Jetting kits and sources:


gen_tuning.gif (11893 bytes)


Data is for 2003 and later and applies to the 640 Adventure and LC4E equipped with a Mikuni BST40 Carb. This info may be used for the '03 SXC, however, the off-road type muffler may alter the final jetting outcome.
Addendum: Additional information has been added for the benefit of 2002 and earlier LC4 owners. It is highlighted in Blue. All other information applies as well.

1. Main jet.
I believe a 152.5 main is still the stock piece. If you are at 3000 ft. elevation or less, a 160.0 main jet is a good place to start. Above that elevation, drop down a size for every 1000 feet or so.
Drain the float bowl using the float bowl drain screw on the bottom left front of the bowl.
Pay attention to the orientation of the spacer that comes out with the jet. This change will improve performance and throttle response at 1/2 throttle and above.
(2002 and earlier bikes have a 142.5 main jet stock... a good place to start there is a 150.0 to 152.5)

The main jet recommended is a starting point. You may find after riding for a time that you will want to try 1 size larger or smaller.
These jets are the same main jets used in Mikuni HSR carburetors for Harley-Davidsons, which makes then available at just about any H-D dealer or custom shop.

2. Removing the vacuum cap from the carburetor.
Remove the fairing and fuel tank, you don't have to remove the dash, simply tie the dashes "ears" up to the handlebars.
Remove the 2 screws holding the cap on while lightly holding the cap down. Remove the cap, and gently remove the vacuum piston/slide assembly. Remove the spring and push the needle up and out the top of the slide.
Note the position of the parts; refer to your owner's parts catalog for correct position.

2. a. Changing the needle clip.
Remove the clip (do this over a large table, maybe with a white bath towel on it... just in case you "loose" the clip) from the 3rd position from the top and relocate it to the 4th position.
For elevation, this may be an option. You may find that the stock position responds well or that rather than shift the clip one full rich position you could install a shim washer that would move the needle 1/2 a groove spacing.
This change will improve performance and throttle response at 1/4 throttle and above.

2. b. Drilling the slide.
Obtain a drill bit, preferably a longish one, .110" to .125" in diameter (3mm is the prefered drill bit size) and drill out the two vacuum ports in the slide, on either side of the needle hole to the drill diameter. Drilling from the top is slightly better because the surface inside the slide is square to the ports; the outside has a radius that requires you to have a bit of drilling talent. This bit works regardless of elevation... it simply allows the vacuum piston to react to throttle changes more rapidly.
This change will quicken throttle response at 1/8 throttle and above.

A note about cutting springs. A few folks like to promote cutting the vacuum piston return spring. I would explain why this is not such a good idea, but I've done it once and that was more than enough.
If you really think this is something you want to do, do a search and read up on it.

2. c. Pilot fuel and air jets.
You probably have a 42 or 45 pilot fuel jet and a 1.2 pilot air jet, and you may have heard that richening this circuit is a good idea. Personally, I have found no improvement by increasing the fuel or decreasing the air jet size. I live at 500 ft. above sea level.
(some 2002 and earlier bikes, those that are not equipped with the "high flow" cylinder head do seem to benefit from a slightly richer pilot circuit)

Again, if you think this is something you want to do, do a search and read up on it. The jets are cheap, so if it doesn't pan out for your location and riding style... no big deal.


That's all the mods... you can reassemble the carb. You can use your finger to help guide the needle back into the needle jet in the carb body, then insert the spring. Be careful when reinstalling the cap that you do not pinch or distort the vacuum diaphragm. Don't over-tighten the cap screws.

3. The air box is easy... either buy or make a copy of the KTM high flow air box side cover. Do not remove the snorkel on top the air box. Bear in mind that if you clean and re-oil the air filter, you bike will run richer than normal until the excess oil on the filter dissipates.
This mod improves air flow to the carburetor without upsetting the pressure balance required for it to work. Removing the snorkel is a recommendation made only when certain carb and exhaust conditions are met.

"584.06.003.200 LC4 (Small Airbox) Better airflow rate"

4. Modifying the stock muffler.
The best part about this is that KTM uses a SuperTrapp USFS diffuser disc spark arrestor system on this bike... easy to tune once you remove a bit of "restriction" downstream.
Remove the Supertrapp spark arrestor discs and end cap. Obtain a 1.5" hole saw, find a wooden dowel that fits tightly in the center hole of the mechanical baffle... you need this to help center the drill.
Core out the baffle tube with the hole saw. A tube of about 6 inches in length comes out when the hole saw cut thru.
Reinstall the discs... the bike probably comes with 10 to 12 discs stock... you can add up to around 16 to suit your personal sound meter, but as I recall, 10 to 12 is more than enough for most folks.
This mod improves air flow thru the muffler, improves performance and makes the bike louder. However, the air and sound output of the muffler can be controlled by subtraction or addition of diffuser discs.

Most aftermarket mufflers are 5 to 7 pounds lighter than the stocker. Most will improve power over the stocker, but few will be a noticeable improvement over the modified stocker... unless you want to spend $1000.
I have a Supertrapp IDS2 with a "quiet core". I did this for three reasons... tuneability, weight and cost.

5. Fine Tuning
All of this is based on the likelihood that you do not have a $100 Motion Pro 90° carb screwdriver, and trying to do this hot and keep track of the number of turns you have on the idle mixture screw is a very difficult thing to do.
If you have asbestos covered hands and tiny fingers, you can disregard all this and tune to your hearts content.

Ride the bike at least 10 minutes to get it up to operating temperature; set the hot idle speed at 1800-2000 rpm. Now allow the bike to cool.

Locate the idle mixture screw at the front bottom of the carb body, carefully screw it in until it lightly bottoms... you will need an exceptionally short screwdriver for this.
Turn the screw out two and one quarter turns (2.25) and ride the bike, note the throttle response and idle speed.
Turn the screw to one and three quarter turns out (1.5) or 3/4ths (.75) in from the previous position and ride the bike, note the throttle response and idle speed.
Of the two positions, one will have slightly better throttle response and faster idle... this is the range you want to work in.

The idea here is to use the screw to optimize the transition from the pilot circuit to the needle circuit... not get a perfect idle mixture. That's why your rpm is temporarily set in that transitional range.
Do you want a spiffy idle... or, do you want the best off-idle throttle response you can get?

In a perfect world, you would be able to adjust the screw while the engine is running and locate an optimum position... but as I said, unless you have the tool I have, you have to do it trial and error.
Once you have the low speed mixture where you want it, reset the idle to 1600-1650 rpm.

Loadedagain has made a quantity of extended screws (like the ZipTy FCR ones) for the BST40. PM him if you want one as he only made maybe 50 of them in a CNC run.
It's handy to be able to optimize the idle mix when you have dramatic changes in elevation... especially for the off-road, just off idle, tight single track stuff.

Last thing... the jetting info in this post is for an '03 to present 640 Adventure
(and pre '03 if you read closely). Not an 640 SM, not a Duke, not a '96 400 with a BST... not even a 640 with a BST and a complete Akrapovic system... OK?

Have fun,


Everything that I've covered should be considered a functional baseline. Most folks bikes run great using the jetting refered to, while some work better going up or down a bit.
It's not rocket science... just trying to find the correct air/fuel ratio for your average elevation and temperature, and intake/exhaust mods.
If you have access to a dyno with an A/F sensor, you'll find that BST equipped LC4s make the smoothest power and have the best throttle response when the A/F is between 13.4:1 and 14.2:1.
They'll run OK in the high 12s and mid 14s... but you tend to waste fuel in one direction and run a bit hot in the other.